There is no easy way to sex chicks at a young age, unless they have sex-linkage or are an autosexing breed (if you cross certain colours you can get different colouring of day old male / female chicks). Some professionals can sex chicks by looking at the vent but this is difficult for the novice and usually comes with experience within certain strains.
To be sure about the sex of your chicks, you normally have to wait until wattles / combs start to develop by 5 to 8 weeks or in some breeds, until they start to crow at 12-14 weeks to be completely sure. The price of pullets is usually reflected in this – i.e. hybrids that are sold at £12-£17 each are autosexing and the supplier can cull at day old. Pure breeds are double the price as they usually are feeding cockerels too until they can tell what is what as well as being slower to mature. An example of an autosexing breed is the Cream Legbar or the Rhodebar. Sexing of these chicks can be done at a day old – see this article on Autosexing for more information.